1. Field of the Invention
The present invention relates to an ink-jet recording apparatus which comprises an electromechanical transducer and a nozzle-having ink chamber and which is so constructed that the electromechanical transducer is driven by a pulse voltage to put pressure upon the nozzle-having ink chamber to thereby eject ink droplets from the nozzle, and more particularly to an on-demand-type ink-jet recording apparatus which is capable of making high-quality gradation and color image recordings.
2. Description of the Prior Art
Firstly, existing ink-jet recording apparatus of the prior art is summarized below:
When pressure is put on a nozzle-having liquid chamber so as to contract the inside volume thereof, the liquid inside the liquid chamber is compressed in the form of fluid droplets from the nozzle provided to the liquid chamber.
If an ink is used as the liquid inside the chamber and a recording medium (e.g., a sheet of recording paper) is provided in front of the nozzle, and if the above operation takes place in accordance with a recording signal (pulse voltage), then the ink droplets ejected from the nozzle strike the recording sheet to thereby record ink dots thereon. In this operation, for example, if a recording sheet is moved in the vertical direction and the nozzle is moved in the lateral direction, then any desired patterns such as character or letter patterns, etc., consisting of ink dots can be recorded on the whole area of a recording sheet.
Such the recording apparatus as in above, as the on-demand type ink-jet recording apparatus, is already on the market. The on-demand-type ink-jet recording apparatus is not one that records ink dots on a recording sheet by the wire impact as in the case of wire-dot-type printers but one in which ink 2 supplied in an ink chamber 3 through an ink supplying passage 4 as in FIG. 1(a) is ejected in the form of ink droplets from nozzle 1, and the droplets strike a recording sheet to thereby form ink dots thereon, so that the recording operation can be carried out very quietly. And mechanically driving means necessary for the recording operation are simple and small in the number thereof, so that the apparatus can be easily constructed of a compact type. Further, as the means for putting pressure upon the ink liquid, if such an electromechanical transducer as, for example, a piezoelectric element 7 is used which is made of piezoelectric crystals such as barium titanate ceramics (under the trade name "PZT" available commercially from Clevite Corporation, Cleveland, Ohio), or the like, then the recording can take place in a short period of time.
That is, the on-demand-type ink-jet recording apparatus is capable of recording information much more noiselessly and faster than does the wire-dot-type printer, and being of a compact construction. Further, a plurality of ink liquids different in color can be used to make superposed printings at same points on a recording sheet to thereby make multicolor recordings comprising not only the respective inks' own colors but also their mixed colors.
In the on-demand-type ink-jet recording apparatus, in order to record high-density and high-resolution information on a recording sheet, it is necessary to minimize the size of each of the dots to be recorded on the recording sheet. For this purpose, the size of each of the ink droplets ejected from the nozzle must be minimized.
If graphical images are to be recorded on a recording sheet, the density needs to be changed by multiple stages. For the multistage change in the density there may be used a method to change the number of ink dots per unit area on a recording sheet. A high-density record can be obtained by increasing the number of ink dots, while a low-density record can be obtained by reducing the number of ink dots. However, the use of this method alone has its limit to the representation of halftone gradation. To change the density by multiple stages, the number of ink dots per unit area on a recording sheet should be varied along with controlling the diameter of each of the ink dots to be recorded on the recording sheet.
Namely, in the on-demand-type ink-jet recording apparatus, in order to record high-density, high-resolution and multistage-density information on a recording sheet, it is desirable that the size of each of the ink droplets ejected from the nozzle be freely controllable at need from much smaller sizes to larger sizes. Minimization of the size of the droplet from the nozzle is considered carried out by making smaller the diameter of the nozzle orifice, but if the diameter of the nozzle orifice is made smaller, the nozzle tends to become clogged with increasing the friction of the ink liquid with the nozzle, so that the ink liquid becomes hardly ejected from the nozzle. For this reason, there is naturally a limit to making small the nozzle orifice. And making small the diameter of the nozzle orifice, although it reduces the size of the ink droplet to a certain extent, cannot freely controll the size.
For increasing the density of information to be recorded on a recording sheet there is also another method, which utilizes "satelite" droplets that are secondarily formed behind and smaller than the main ink droplets when ejected from the nozzle. The main ink droplets and satelite ink droplets are ejected in the same direction from the nozzle, so that the points on a recording sheet where these droplets strike are the same if no manipulation is applied thereto. In order to change the size of the ink droplet to be recorded on a recording sheet, the main droplets and the satelight droplets should be properly used separately, and the satelite droplets alone must be used for the small-size dot recording with a manipulation to prevent the main droplets from arriving at the recording sheet. For this reason, the apparatus requires means for charging the main droplets to deflect the same and a device for the recovery of the unused main droplets, thus causing the apparatus to become of a large size. Since the satelite droplets are ones secondarily produced when the main droplets are ejected from the nozzle, the size thereof cannot be freely controlled. The diameter of each of the ink dots to be formed on a recording sheet, although there is a change in the size due to the difference between the main droplets and the satelite droplets, cannot be variably controlled.
Subsequently, in an attempt to freely change the size of the ink droplet to control the size of each of the ink dots to be formed on a recording sheet there was devised a device for changing the magnitude (height) of a pulse voltage applied to an electromechanical transducer to change pressure put on the ink liquid inside the ink chamber to thereby control the size of the ink droplet from the nozzle. This is an attempt to control the ink droplet size according to the pulse voltage level and it is based on the idea that if pressure to be put on the ink liquid is larger, then the droplet size from the nozzle is larger, while if the pressure is smaller, then the droplet size from the nozzle is smaller.
However, according to this device, the ink droplet size-changeable range is narrow, and it has been found that it is difficult for the device to form ink droplets of a certain size. The ink chamber, electromechanical transducer, and the like, which constitute the ink-jet printer head, have their own intrinsic oscillation frequencies. If the oscillation frequency produced by the pulse voltage applied to the electromechanical transducer is not coincident with the foregoing intrinsic oscillation frequency, then the applied pressure causes ink droplets to be efficiently ejected in a uniform size from the nozzle, but if the oscillation frequency produced by the pulse voltage applied to the electromechanical transducer is close to resonance frequency, then the oscillation frequency is attracted to the resonance frequency, whereby the ink droplet ejection from the nozzle becomes unstable. This is considered to be the cause of narrowing the ink droplet size-changeable range. Accordingly, even if the pulse voltage to be applied to the electromechanical transducer were changed to thereby change pressure put on the ink liquid, the ink droplet size from the nozzle would be unable to be changed arbitrarily, so that the dots to be formed on a recording sheet is not controllable freely.
As has been described, existing techniques of the prior art are unable to control freely the size of the droplet ejected from the nozzle, and thus unable to control the size of the dot to be formed on a recording sheet.
As a result of our investigation, we have found out that even where the same pulse voltage is applied to the electro-mechanical transducer, the tip position of the ink liquid inside the nozzle at the time when the pulse voltage is applied has relation to the size of the droplet.
Namely, even if the same pulse voltage were applied to the electromechanical transducer, the droplet ejected from the nozzle would become different in the size between when nozzle 1 is filled with an ink liquid 2 so that the tip end position of ink liquid 2 comes up to the orifice of nozzle 1 as shown in FIG. 1(b) and when the tip position of ink liquid 2 inside nozzle 1 is at a distance l from the orifice of nozzle 1 as shown in FIG. 1(c).
If the size of the ink droplet is taken on the axis of ordinate and the distance l between the nozzle orifice and the tip position of the ink liquid is taken on the axis of abscissa, even when the same pulse voltage is applied, the ink droplet size changes as given in FIG. 2.